The combustion space of a power station boiler is typically 100 ft by 100 ft in plan and has a multiplicity of steam tubes passing therethrough. A leak of steam from one of the tubes can cause a drop in efficiency of the boiler and can result in secondary damage to the boiler tubes and also to the boiler roof. But at present there is no direct way of monitoring for the presence of a steam leak. It is known that escape of steam results in an increased noise level in the boiler, but locating a microphone within the combustion space is impractical because of the high ambient temperatures and also because of the acidic and dusty atmosphere.